This invention relates to a corona effect surface treatment apparatus for sheet materials.
The corona effect surface treatment of materials is known to consist in causing a corona discharge in the proximity of the surface of the material to be treated in order to achieve surface adhesion characteristics which are useful for anchoring ink, glue, and the like thereto.
The known apparatus intended for corona effect surface treating materials comprise essentially paired parallel electrodes, one of which is coated with an insulating material and the other is of metal; such electrodes are connected to an electric generator operating at a voltage and frequency such as to produce a discharge distributed along the whole length of the electrodes at their mutually facing areas.
The materials to be treated which may be sheets or foils of plastics and other materials are inserted between the two electrodes in the area where the discharge is to occur. Since nearly all of the electric power supplied to the electrodes is converted into heat, the latter is distributed between the surfaces involved by the discharge, thus raising the electrode temperature.
A reduction of the residence time of the material being treated under the discharge, as required for a high production rate or output, requires an increase of the discharge density over the electrodes in order to maintain constant the energy required for the required degree of surface treatment.
This results in an increase of the thermal energy dispensed to the stationary electrodes and a consequent increase of temperature, with an attendant deterioration of the dielectric properties of the insulated electrode.
That difficulty has been obviated in the past in one of two ways:
1. By removing the heat developed on the insulated electrode through the material to be treated, which was caused to slide in intimate contact therewith; PA1 2. By introducing to the inside of the insulated electrode a heat conductive metal bar capable of dissipating it to fins arranged on the heads.
Either methods have applicational limitations when employed in conjunction with high discharge densities; the method (1) owing to the dielectric material of the insulated electrode overheating at the side areas which are not enveloped by the film, the method (2) owing to the relatively small subtraction of heat obtained through conduction by the central bar, and the longitudinal temperature diagram profile of the insulated cylinder, which exhibits a higher temperature in the middle area with attendant higher emissivity and discharge concentration.